Answer
$(m+15)(m-15)$
Work Step by Step
$m^{2}-225=$
...rewrite $225$ as a square.
$=m^{2}-15^{2}$
...factor using the rule for a difference of two squares.
($a^{2}-b^{2}=(a+b)(a-b),\ a=m,\ b=15$)
$=(m+15)(m-15)$
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